1. Field of the Invention
The present invention relates to a switching control circuit.
2. Description of the Related Art
With respect to a switching power supply circuit configured to generate an output voltage of a target level from an input voltage, a power supply circuit of a type called a ripple converter or a hysteresis control regulator (see, e.g., Japanese Laid-Open Patent Publication No. 2004-104942) is known. FIG. 20 depicts one example of a ripple converter 300 of a common fixed-on-time system. When a feedback voltage Vfb corresponding to an output voltage Vout decreases to be lower than a reference voltage Vref, a control circuit 410 turns on an NMOS transistor 420 for a predetermined time. As a result, the output voltage Vout increases. When the output voltage Vout is decreased by an effect of a load and the feedback voltage Vref becomes lower than the reference voltage Vref, the control circuit 410 again turns on the NMOS transistor 420 for the predetermined time. Such an operation is repeated, thereby generating the output voltage Vout of the target level.
Incidentally, in the ripple converter 300, a ceramic capacitor 440 with small ESR (Equivalent Series Resistance) is occasionally employed to reduce a ripple voltage of the output voltage Vout. In such a case, the ripple voltage contained in the feedback voltage Vfb becomes small, which may leads to unstable operation of the ripple converter 300. Thus, in order to operate the ripple converter 300 in a stable manner, an adjusting circuit (not shown) which adjusts a gain and a phase of the feedback voltage Vfb and a circuit (not shown) that is configured to generate the ripple voltage based on a current flowing through an inductor 430 and adds the generated voltage to feedback voltage Vfb are used, for example (see Japanese Laid-Open Patent Publication No. 2004-104942). However, since the values of elements such as a resistor, a capacitor, etc., included in these circuits is generally large, the adjusting circuit, etc., cannot be included in the control circuit 410 when the control circuit 410 is integrated, thereby increasing the number of external components of the ripple converter 300.